Digital audio stereo imager

ABSTRACT

An apparatus and a method for controlling, during playback, the composition of a pair of stereo separated audio signals in a stereo sound entertainment system by changing the stereo image width are disclosed. A digital mixer combines a portion of a first digital audio signal with a portion of a second digital audio signal to create a first mixed signal and a portion of the second digital audio signal with a portion of the first digital audio signal to create a second mixed signal. A listener controller independently controls, in accordance with a listener input, the portion of the first digital audio signal combined with the portion of the second digital audio signal and the portion of the second digital audio signal combined with the portion of the first digital audio signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.61/138,655, filed Dec. 18, 2008, which is incorporated herein byreference in its entirety.

BACKGROUND

Much of modern audio recording and listening is based on stereophonicsound, commonly called “stereo” or “surround sound.” Stereophonic soundis the reproduction of sound, using two or more independent audiochannels, through a symmetrical configuration of loudspeakers(speakers), in such a way as to create a pleasant and natural impressionof sound heard from various directions, as in natural hearing. A stereosystem offers the capability of reproducing at least some “sense ofspace,” thus providing a listener with a more realistic reproduction ofan original acoustic performance. Employing additional audio channelsmay enhance that capability. In popular usage, stereo usually meanstwo-channel sound recording and sound reproduction using data for morethan one speaker simultaneously. In technical usage, “stereo” or“stereophony” means sound recording and sound reproduction that usesstereographic projection to encode the relative positions of recordedobjects and events. A stereo system can include any number of channels,such as the surround sound 5.1- and 6.1-channel systems. However, incommon use it refers to systems with only two channels.

Normally, preparation of audio material for commercial distribution isdone in a special listening environment, such as a mixing studio ormastering studio. The listening environment is typically equipped withaudio speakers. An audio engineer can make many types of sonicadjustments while listening to the sound coming out of the speakers. Onevery important adjustment that can be made is called “sound separation,”stereophonic sound attempts to create an illusion of location forvarious instruments within the original recording. The recordingengineer's goal is usually to create a stereo “image” with localizationinformation.

When playing back stereo recordings, the best results are obtained byusing two speakers, located in front of and equidistant from thelistener, with the listener located on the center line between, andfacing, the two speakers. The listener's left ear has a more direct pathto the left, and the right ear has a more direct path to the rightspeaker. When a stereophonic recording is heard through loudspeakersystems rather than headphones, each ear hears sound from both speakers.As in the case of a live musical performance, both ears react to soundsfrom all directions. Much of the sense of space is due to the relativeamount of sound received by the left and right ear. An audio engineermay and often does use more than two microphones, sometimes many more,mixing the microphone signals down to two tracks. Typically, each audiostereo channel carries some of its own individual sound and also somesound that is common to both channels.

If all the sound in both channels is common, the system becomes amonophonic system resulting in loss of spatial perception. If thechannels are totally separated, for example, when a listener is usingheadphones, proper spatial perception is lost. The resulting effect isdistorted spatial perception. Headphones, therefore, introduceadditional, unintended separation between the audio sound heard bylistener's left and right ears. A signal out of a left speaker isintended to reach both ears, but with headphones, the same signalreaches only the left ear. Similarly, a signal out of a right speaker isintended to reach both ears, but with headphones, the same signalreaches only the right ear. Therefore, headphones result in addedunintended stereo separation.

Significant differences between a recording environment, for example, amusic production studio loudspeaker setup and a home listeningenvironment setup, can also cause alterations between the intendedspatial perception and the actual spatial perception in the homelistening environment. Many home listening environments are set up in away that is far from perfectly emulating an ideal listening studiostereo setup. Many factors, such room size, doors, windows, furniturelocation, etc., may cause a home listener to be located in a less thensymmetrical listening position relative to the speakers. Thus, thespatial perception intended by a recording engineer in a recording(e.g., production studio) environment can be distorted in a homelistening environment due to the differences between the studio and thehome listening environments.

Therefore, there is a need for a system and method that allow a listenerto control the composition of stereo separated audio signals in a soundentertainment system so as to enhance the listener's enjoyment bycompensating for the distortions caused by the differences betweenrecording and listening (e.g., home) environments.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

An apparatus and a method for controlling, during playback, thecomposition of a pair of stereo separated audio signals in a stereosound entertainment system by digitally changing the stereo image widthare disclosed. The method and apparatus enhance an end user's(listener's) enjoyment by allowing the user to digitally mix the pair ofstereo separated audio signals in a manner that compensates for thedifference between a recording (e.g., studio production) environment andthe user's listening environment. More specifically, during playback, aportion of one of the pair of stereo separated audio signals isdigitally mixed with a portion of the other of the pair of stereoseparated audio signals and vice versa prior to the resulting digitallymixed stereo audio signals being presented to the end user (listener).

In one exemplary illustrative embodiment, a stereo sound entertainmentsystem apparatus for controlling, in accordance with a listener input,the composition of a pair of stereo separated audio signals isdisclosed. The apparatus comprises: a digital mixer configured toreceive two stereo separated, digital, audio signals, combine a portionof a first audio signal with a portion of a second audio signal tocreate a first mixed signal, and combine a portion of the second audiosignal with a portion of the first audio signal to create a second mixedsignal; and a listener controller coupled to the mixer and configured toindependently control, in accordance with a listener input, the portionof the first audio signal combined with the portion of the second audiosignal and the portion of the second audio signal combined with theportion of the first audio signal.

In one exemplary illustrative embodiment, the digital mixer comprises amultiplier arrangement and channel mixers. The multiplier arrangement isconfigured to: combine a portion of the first audio signal with theportion of a second audio signal by scaling the first audio signal by afirst scaling factor and scaling the second audio signal by a secondscaling factor related to the first scaling factor; and combine aportion of the second audio signal with a portion of the first audiosignal by scaling the second audio signal by a third scaling factor andscaling the first audio signal by a fourth scaling factor related to thethird scaling factor. The channel mixers mix the scaled audio signals tocreate the first mixed signal and the second mixed signal.

In another exemplary illustrative embodiment, the digital mixercomprises a computer processor and a memory having computer-executableinstructions stored thereon, which, when executed by the computerprocessor, cause the computer processor to combine a portion of thefirst audio signal with a portion of the second audio signal to createthe first mixed signal and combine a portion of the second audio signalwith a portion of the first audio signal to create the second mixedsignal.

In yet another exemplary illustrative embodiment, a method forcontrolling the composition of a pair of stereo separated audio signalsin a stereo sound entertainment system in accordance with listener inputis disclosed. The method comprises receiving two stereo separated,digital, audio signals and, in response to listener input, combining aportion of a first audio signal with a portion of a second audio signalto create a first mixed signal, and combining a portion of the secondaudio signal with a portion of the first audio signal to create a secondmixed signal.

As will be readily appreciated from the foregoing summary, systems andmethods that allow a listener to control, during playback, thecomposition of stereo separated audio signals in a sound entertainmentsystem so as to enhance the listener's enjoyment by compensating for thedistortions caused by differences between recording and listeningenvironments are provided. While the systems and methods are ideallysuited for use in a stereo headphone environment, the methods andsystems may also find use in stereo speaker environments, both dualchannel and multiple channel (i.e., surround sound) stereo speakerenvironments. The systems and methods enhance listener enjoyment bychanging stereo image width. Depending on how a user controls thecombination of the pair of stereo separated audio signals, the width ofthe resulting mixed signals can be increased or decreased.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating an exemplary embodiment of anapparatus for controlling the composition of a pair of stereo separatedaudio signals in a stereo sound entertainment system;

FIG. 2 is a functional diagram illustrating an exemplary routineperformed by a mixer configured to decrease, i.e., reduce, stereo imagewidth;

FIG. 3 is a functional diagram illustrating an exemplary routineperformed by a mixer configured to increase stereo image width;

FIG. 4 is a block diagram illustrating another embodiment of anapparatus for controlling the composition of a pair of stereo separatedaudio signals in a stereo sound entertainment system;

FIG. 5 is a functional flow diagram illustrating an exemplary routinefor controlling the stereo image width suitable for use by the systemillustrated in FIG. 4; and

FIG. 6 is a pictorial diagram illustrating a front panel of an exemplarystereo sound entertainment system that incorporates an apparatus forcontrolling the composition of a pair of stereo separated audio signals.

DETAILED DESCRIPTION

As discussed above, listening to a stereo recording through headphonesis often not as enjoyable as it could be due to the separation betweenthe audio signals coming through “left” and “right” headphones. Thisundesirable playback result occurs because the left audio signal iscompletely isolated from the right audio signal and vice versa. Thoseskilled in the art will appreciate that “left” and “right” are usedherein for ease of description and understanding only. This should betaken as exemplary and not limiting because, as those skilled in the artwill appreciate, while the described systems and methods are ideallysuited for use in a stereo headphone listening environment, the systemsand methods may also find use in a speaker stereo environment, includingspeaker stereo environments that include more than two speakers, such assurround sound stereo speaker environments. Regardless of the listingenvironment, stereo sound distortion occurs during playback because thelistening environment does not perfectly match the recordingenvironment.

For the best playback listening enjoyment, each “channel” of a stereolistening environment (both headphone and speaker) should carry a mix ofthe channel's “own sound” and sound that is common to both channelsblended in a way that recreates the “sense of space” of the originalrecording environment. In order to create the proper “sense of space”for a listener, it is necessary to be able to control the stereo imagewidth of the audio signals in each channel. As will be better understoodfrom the following description, this is accomplished by allowing alistener to control the ratio between common and individual sounds ineach channel. In effect, changing the ratio between common andindividual sounds in each channel controls the composition of the audiosignal in each stereo sound channel.

Changing stereo image width is accomplished by electronically injecting(adding or subtracting) under listener control some of the signal fromthe right channel into the left channel and vice versa. That is, auser-selected portion of the signal that is common to both channels isadded to, or subtracted from, each individual channel signal, in orderto narrow or increase the stereo image width. This approach allows auser to compensate for the wide variation in taste between musicproducers, as well as variations in studio stereo setups and homesetups.

The audio signal composition manipulation described above isaccomplished by manipulating digital signals using appropriate hardwareor software. More specifically, as more fully described below, duringplayback the audio digital signals in a stereo system, before theirconversion into analog form and sent to earphones or loudspeakers, arealtered under user control to create the desired stereo image width.Briefly, the stereo image width of each channel is controlled bycreating intermediate mixed signals by adding a portion of the digitalsignal of one channel to the digital signal of the other channel or bysubtracting a portion of the digital signal of the one channel from thedigital signal of the other channel. The added or subtracted portion iscontrolled by the user. While both channel signals could besimultaneously adjusted, preferably each channel signal is separatelyadjusted. Scaling is used to maintain the resulting channel signalswithin a range that avoids distortion.

Stereo Image Width Reduction

Assume that L is the portion of a digital signal that appears only onthe left channel, R is the portion of the digital signal that appearsonly on the right channel, and C is the portion of the digital signalthat is common to both channels. Thus, the left channel signal isLeft=L+C. The right channel signal is Right=R+C. Adding the left andright channel signal results in a SUM signal, namely,SUM=Left+Right=L+R+2° C. The SUM signal comprises two parts:

a. The (L+R) part is the portion containing the sum of the individualchannel signals.

b. The (2*C) part is the portion of the signal common to both channels.

The SUM signal contains no separate L and R parts and represents ascaled monophonic signal that may be called a new common signal. Thescaling factor depends on the original stereo signal separation.

Adding a portion KL of the SUM signal to the left channel signal, i.e.,L+C, yields a new left channel signal, namely, NewLeft=L+C+KL*SUM=L+NewCL. Thus, the NewCL signal comprises the originalcommon signal, C, and the additional common signal KL*SUM. In summary,the NewLeft signal comprises the same amount of separate signal L but anincreased amount of a common signal, namely, NewCL.

Similarly, adding a portion KR of the SUM signal to the right channelsignal i.e., R+C, yields a new right channel signal, namely, NewRight=R+C+KR*SUM=R+NewCR. NewCR comprises the original common signal, C,and the additional common signal KR*SUM. Thus, the New Right signalcomprises the same amount of separate signal R but an increased amountof a common signal, namely, NewCR.

In summary, the summation of the left and right channel digital signalsyields a new common signal. Adding a desired portion of the new commonsignal to each channel “dilutes” the ratio of the separate portion (L orR) in each channel. Lowering the portions of separate L and R signalsreduces the stereo separation. The amount of the SUM signal added to theleft channel signal is controlled by the scaling factor KL, and theamount of the SUM signal added to the right channel signal is controlledby the scaling factor KR. While both channel signals can be controlledsimultaneously and equally, by making KL=KR, preferably KL and KR areseparately controlled by the listener.

Stereo Image Width Increase

As in the above example, assume L is the portion of the digital signalthat appears only on the left channel, R is the portion of the digitalsignal that appears only on the right channel, and C is the portion ofthe digital signal that is common to both channels. Thus, again, theleft channel signal is Left=L+C and the right channel signal isRight=R+C. The two channel signals can be subtracted from another tocreate a left channel difference signal (Diff_LR) and a right channeldifference signal (Diff_RL) that eliminates the common portion of thesignals. More specifically:

Diff_(—) LR=(L+C)−(R+C)=L−R.

Diff_(—) RL=(R+C)−(L+C)=R−L.

Adding a fraction KL of Diff_LR to the left signal yields a new leftchannel signal (New Left). More specifically, NewLeft=(L+C)+KL*(L−R)=(1+KL)*(L)+(C−KL*R). As a result, the separateportion of the left signal, i.e., L, is increased by a factor (1+KL).Also, the original common signal C is decreased by KL*R. Therefore, theproportion of L in the New Left signal has increased.

Similarly, adding a fraction KR of Diff_RL to R increases the portion ofthe separate signal R in a New Right signal.

In summary, subtractions of the left channel digital signal from theright channel digital signal and the right channel digital signal fromleft channel digital signal yield a pair of intermediate digitalsignals. Adding a portion of those signals to each channel increases theseparate portion in each channel (R or L). The greater R and L portionsof the respective channel signals enhance stereo sound separation.

Scaling

To avoid clipping, signals that are generated as a result of addition orsubtraction of portions of signals should not exceed an allowed range.Given that the left and right channel signals alone may reach themaximum allowed signal value, summing portions of one channel signalwithin the other channel signal may exceed the allowed signal value. Inorder to prevent the resulting signal from exceeding the allowed signalvalue, it is necessary to scale down the components of the left andright channel signals.

Simply scaling, i.e., reducing a signal, such as a digital audio signal,below a certain value is undesirable because such scaling lowers thesignal to noise ratio. For example, reduction of a signal by a factor of2 is a loss of 1 bit (lowering the signal to noise ratio by 6.02 dB). Areduction by a factor of 4 is a loss of 2 bits (lowering the signal tonoise ratio by 12.0 4 dB).

Better scaling yields a modified image width alteration signal that isnear the original signal strength. Doing so maximizes the signal tonoise ratio, which is one of the goals for a high quality audio system.If each signal component (Left or Right) can have a value as high asVmax and as low as Vmin, both Left and Right must be scaled down by afactor of 2. The scaled addition yields SUM=1/2*Left+1/2*Right. Eachresulting signal then will range between 1/2Vmax and′ 1/2Vmin, ensuringthat the sum will not exceed the available minimum to maximum range.Such scaling is sufficient to make sure that all the signals(intermediary and final signals) fit within the given range (Vmin toVmax).

Exemplary Embodiments

Exemplary embodiments of systems for controlling stereo image width aredescribed next. While the following description includes numerousspecific details intended to provide a thorough understanding of thedescribed embodiments, as will be apparent to those skilled in the artand others, some of these details may not be included in otherembodiments and/or other details added.

FIG. 1 is a block diagram illustrating an exemplary control system 10for controlling the composition of a pair of stereo separated audiosignals in a stereo sound entertainment system. FIG. 1 also illustratesthe flow of digital signals as they pass through control system 10. Thecontrol system 10 comprises four multipliers 12, 14, 16, and 18, a rightmixer 20, a left mixer 22, a listener controller 32 and twodigital-to-analog converters (DAC) 24 and 26. The control system 10produces two outputs, which are separately applied to two earphones (orspeakers) 28 and 30. As those skilled in the art and others willappreciate, the designation “left” and “right” are used for illustrativepurposes only and should not be construed as limiting. As will also beunderstood by a person skilled in the art, the illustrated components ofthe control system 10 should not be construed as limiting sinceadditional elements, such as, for example, amplifiers and filters, maybe included in an actual embodiment. Further, the number of DACs in anactual embodiment may be more than the two depicted in FIG. 1. Also, asingle multiplier unit that performs the functions of the fourillustrated multipliers 12, 14, 16, and 18, or a single mixer thatperforms the functions of the two depicted mixers 20 and 22, may beincluded in an actual embodiment of the system shown in FIG. 1.

In accordance with user (listener) input, preferably manual input, thelistener controller 32 generates the scaling factors KL and KR, whichare applied to first and second multipliers 12 and 14, respectively.Based on the values of KL and KR established by the listener, thelistener controller 32 also generates the value 1−KL, which is appliedto the third multiplier 18, and the value 1−KR, which is applied to thefourth multiplier 16.

The left channel digital stereo signal, L+C, forms the second input tothe first multiplier 12 and the right channel digital stereo signal,R+C, forms the second input to the second multiplier 14. The first andsecond multipliers 12 and 14 multiply the L+C signal by KL, and the R+Csignal by KR, respectively. The first and second resulting signals aresupplied to the left mixer 22 and the right mixer 20, respectively. Theright channel signal, R+C, is also applied to the other input of thefourth multiplier 18, and left channel signal, L+C, is applied to theother input of the third multiplier 16. The signal that results frommultiplying 1−KL and R+C, i.e., the output of the fourth multiplier 18,is applied to the left mixer 22, and the signal that results frommultiplying R+C and 1−KL, i.e., the output of the third multiplier 16,is applied to the right mixer 20. The outputs of the left and rightmixers 22 and 20 are separately applied to the inputs of the twodigital-to-analog converters 24 and 26. The outputs of the twodigital-to-analog converters are applied to the left and right earphones(or speakers) 28 and 30, respectively, via suitable power amplifier andfilter circuitry.

FIG. 2 illustrates the mathematical functions performed by the left andright mixers 22 and 20 when the width of a stereo image is beingreduced. Block 42 illustrates the two signals received by the left mixer22, i.e., (L+C)*KL and (R+C)*(1−KL), and block 46 illustrates the twosignals received by the right mixer 20, i.e., (R+C)*KL and (L+C)*(1−KR).Blocks 44 and 48 illustrate the mathematical functions performed by theleft and right mixers to produce the left and right channel digitaloutputs that are applied to the DACs 24 and 26, respectively, namely,(L+C)*KL+(R+C)*(1−KL)=L*KL+C*KL+R-R*KL+C-C*KL=L*KL+R*(1−KL)+C and(R+C)*KR+(L+C)*(1−KR)=R*KR+C*KR+L−L*KR+C−C*KR=R*KR+L*(1−KR)+C.

Similarly, FIG. 3 illustrates the mathematical function performed by theleft and right mixers 22 and 20 when the width of a stereo image widthis being increased. As with FIG. 2, blocks 62 and 66 illustrate thesignals received by the left and right mixers 22 and 20, respectively,namely, (L+C)*KL and (R+C)*(1−KL), and (R+C*KR and (L+C)*(1−KR). Blocks64 and 68 illustrate the mathematical functions performed by the leftand right mixers to produce the left and right channel digital outputsthat are applied to the DACs 24 and 26, respectively, namely(L+C)*KL−(R+C)*(1−KL)=L*KL+C*KL−R+R*KL−L+L*KL=L*KL+C (2*KL−1)−R*(1−KL)and(R+C)*KR−(L+C)*(1−KR)=R*KR+C*KR−L+L*KR−C+C*KR=R*KR+C(2*KR−1)−L*(1−KR).

Returning to FIG. 1, as noted above, the output signals of left andright mixers 22 and 20 (illustrated by blocks 44 and 48 in case ofreducing the stereo image width and illustrated by blocks 64 and 66 inthe case of increasing stereo image width) are applied to the left andright digital-to-analog converters 24 and 26.

Those skilled in the art will appreciate that the essentially hardwareembodiment illustrated in FIG. 1 can also be performed by a softwareprogram executed by a computer processing device. FIGS. 4-5 illustratesuch an alternative embodiment, i.e., an exemplary embodiment whereinthe digital signal manipulation described above is performed by softwarestored in the memory of a computing device when executed by a processor.

FIG. 4 illustrates an exemplary embodiment of a stereo system thatincorporates a computing device 80 for controlling stereo image width.The computing device comprises a processor 84 coupled to a memory 82suitable for storing a program that performs the functions illustratedin FIG. 5 and described below when the program is executed by theprocessor. Those skilled in the art will recognize that a variety ofcomputer-readable storage medium may be used to form the memory—randomaccess memory (RAM), flash memory, etc. A listener controller 86provides a user (listener) interface to the processor 84. The listenercontroller is used by the listener to set the values of the scalefactors KL and KR. The processor also receives the left channel signal,L+C, and the right channel signal, R+C. Signal transformation iscontrolled by the software program stored in the memory 82 when theprogram is executed by the processor 84. The resulting left and rightchannel signals are applied to left and right digital-to-analogconverters (DACs) 88 and 90.

FIG. 5 is a functional flow diagram 100 that illustrates how the leftand right channel digital signal are manipulated by the processor 84when the software program stored in the memory 82 is executed by theprocessor. At block 102 the processor receives the left channel digitalsignal L+C and the right channel digital signal R+C. Next, at decisionblock 104 the processor determines the values of the scaling factors KLand KR set by the listener. These values are stored in the memory 82(FIG. 4). The values of KL and KR determine subsequent processing eitherin parallel, as shown, or in series. Regardless of the order, at block108 the process determines which type of signal calculation should beperformed on the right channel signal, i.e., whether the right channelimage width should be increased or decreased If the value of RLindicates that image width should be reduced, at blocks 118 and 120 theright channel signal is modified, i.e., reduced, in accordance with thepreviously described equations [KR*(R+C)+(1−KR*(L+C))] to create thesignal R*KR+L*(1−KR)+C, which is applied to the right channeldigital-to-analog converter 90 (FIG. 4).

If the test at block 108 determines that the right channel signal widthis to be increased, at blocks 154 and 156 the right channel signal isreduced in accordance with the previously described equations[KR*(R+C)+(1−KR)*(L+C)] to create the signal R*KR+

C(2*KR−1)−2*(1−KR), which is applied to the right channeldigital-to-analog converter 90.

At block 110, the process determines which type of calculation should beperformed in the left channel signal, i.e., whether the left channelimage width should be increased or decreased. If the value of KLindicates that the left channel image width should be decreased (i.e.,reduced), the left channel signal is modified, i.e., reduced inaccordance with the previously described equation [KL*(L+C)+(1−KL*(R+C)]to create the signal L*KL+(1−KL)+C, which is applied to the left channeldigital-to-analog converter 88 (FIG. 4).

If the test at block 110 determines that the left channel signal widthis to be increased, at blocks 148 and 150 the left channel signal isincreased in accordance with the previously described equation[KL*(L+C)+(1−KL)*(R+C)] to create the signal L*K*L+C (2*KL−1)−R*(1−KL),which is applied to the left channel digital-to-analog converter 88.

As previously described, the outputs of the left and rightdigital-to-analog converters 88 and 90 are applied to left and rightearphones (or speakers) 92 and 94.

FIG. 6 illustrates an exemplary front panel 200 of a stereo system thatincorporates a control system for controlling the composition of a pairof stereo separated audio signals as described above. Left and rightmomentary contact toggle switches 202 and 204 are used to set thewidening (increasing) or narrowing (reducing) of the left and rightaudio channels. As those skilled in the art will appreciate, a number ofdifferent ways to set the widening and narrowing of stereo image otherthan the way shown in FIG. 6 and described herein can be employed. Forexample, an infrared (“IR”) remote control device (not shown) can beemployed to set the stereo system 200 via an IR receiver 206 shown byway of example at the right side of the front panel 200.

In the case of the illustrated toggle switch control embodiment, eachtoggle of the right channel toggle switch 204 to the right causes thenext indicator lamp 208 of a right set of indicator lamps to the nextright position to light and the prior indicator lamp to go off, untilthe widest position is reached. Each toggle of the right channel toggleswitch 204 to the left causes the next indicator lamp 208 to the nextleft position to light and the prior indicator lamp to go off, until thenarrowest position is reached. In this regard, −3, −1, −1, 0, +1, and +2right relative indicator lamps are illustrated. Obviously, more or lessrelative indicator lamps can be used in other embodiments of thisinvention. The numbers are relative in the sense that they are stepfunctions that do not, per se, identify specific values. As noted by the“wide narrow” terminology over the right toggle switch 204 when comparedto the right channel −3, −2, −1, 0, +1, and +2 lamp identifiers, the −3,−2, and −1 lamp indicators denote a relative narrowing of the rightchannel image, 0 is neutral (no widening or narrowing), and +1 and +2indicate a relative widening of the right channel image.

Each toggle of the left channel toggle switch 202 to the left causes thenext indicator lamp of a left set of indicator lamps 210 to the nextleft position to light and the prior indicator lamp to go off, until thewidest position is reached. Each toggle of the left channel toggleswitch to the right causes the next indicator lamp 210 to the next rightposition to light and the prior indicator lamp to go off, until thenarrowest position is reached. In this regard, as with the rightrelative indicator lamps, +2, +1, 0, −1, −2, and −3 left relativeindicator lamps are illustrated. Obviously, more or less relativeindicator lamps can be used in other embodiments of the invention. Thenumbers are relative in the sense that they are step functions that donot, per se, identify specific values. As noted by the “narrow wide”terminology over the left toggle switch 202, when compared to the leftchannel +2, +1, 0, −1, −2, and −3 lamp identifiers, the −3, −2, and −1lamp indicators denote a relative narrowing of the left channel image, 0is neutral (no widening or narrowing), and +1 and +2 indicate a relativewidening of the left channel image.

As noted above, the numeric indicator values (+2, +1, 0, −1, −2 and −3)are relative values that identify specific add/subtract SUM factors forcontrolling stereo image width. The factors represented by the relatedlit indicator are stored in memory for use by the herein describedembodiments of the inventions.

In addition to the left and right toggle switches 202 and 204 and theleft and right sets of indicator lamps 210 and 208, the front panel 200also has a volume display 212 that displays a number that represents theoutput volume setting of the stereo system. The volume setting iscontrolled by an up/down toggle switch 214. The front panel 200 alsoincludes a headphone jack 216 for connecting the stereo system to theleft/right earpieces of a headphone and a power button 218 forcontrolling the on/off state of the stereo system. As denoted byindicator arrows located above the toggle switches, one of the toggleswitches is a three-position toggle switch whose state determines whichof the left and right channels is enabled for changing when the toggleswitch is up or down, the center position being a neutral (noenablement) position.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention. For example, thetoggle switches can be replaced by push button or rotary switches. Or,as noted above, more or fewer than six relative wide/narrow indicatorsand related values can be used in an actual embodiment of the invention.If desired, rather than incremental values, a continuous spread ofvalues can be provided in a rotary dial switch embodiment, for example.Hence, within the scope of the appended claims, it is to be understoodthat the invention can be practiced otherwise than as specificallydescribed herein.

1. In a stereo sound entertainment system, apparatus for digitallycontrolling, during playback, in accordance with listener input, thecomposition of a pair of pre-recorded stereo separated audio signals,the apparatus comprising: (a) a digital mixer configured to: (i) receivetwo pre-recorded stereo separated, digital audio signals; (ii) combine aportion of a first digital audio signal of the two pre-recorded stereoseparated digital audio signals (“first audio signal”) with a portion ofa second digital audio signal of the two pre-recorded stereo digitalaudio signals (“second audio signal”) to create a first mixed signal;and (iii) combine a portion of the second digital audio signal with aportion of the first digital audio signal to create a second mixedsignal; and (b) a listener controller coupled to said digital mixerconfigured to independently control, in accordance with listener input:(i) the portion of the first digital audio signal combined with theportion of the second digital audio signal; and (ii) the portion of thesecond digital audio signal combined with the portion of the firstdigital audio signal.
 2. The apparatus of claim 1, wherein: (a)combining a portion of a first digital audio signal with a portion of asecond digital audio signal to create a first mixed signal comprises:(i) scaling the first audio signal by a first scaling factor; (ii)scaling the second audio signal by a second scaling factor related tothe first scaling factor; and (iii) mixing the scaled audio signals tocreate the first mixed signal; (b) combining a portion of the seconddigital audio signal with a portion of the first digital audio signal tocreate a second mixed signal comprises: (i) scaling the second audiosignal by a third scaling factor; (ii) scaling the first audio signal bya fourth scaling factor related to the third scaling factor; and (iii)mixing the scaled audio signals to create the second mixed signal; and(c) the magnitudes of the first and third scaling factors are controlledby the listener controller in accordance with listener input.
 3. Theapparatus of claim 2, wherein the second scaling factor is related tothe first scaling factor by subtracting the first scaling factor from afirst predetermined value.
 4. The apparatus of claim 2, wherein thefourth scaling factor is related to the third scaling factor bysubtracting the third scaling factor from a second predetermined value.5. The apparatus of claim 2, wherein mixing the scaled audio signalscomprises adding the scaled audio signals to each other.
 6. Theapparatus of claim 2, wherein mixing the scaled audio signals comprisessubtracting the scaled audio signals from each other.
 7. The apparatusof claim 1, wherein the digital mixer comprises: (a) a first multiplierfor multiplying the first audio signal by a first scale factor; (b) asecond multiplier for multiplying the second audio signal by a secondscale factor; (c) a first channel mixer for mixing the outputs of thefirst and second multipliers to create the first mixed signal; (d) athird multiplier for multiplying the second audio signal by a thirdscale factor; (e) a fourth multiplier for multiplying the first audiosignal by a fourth scale factor; and (f) a second channel mixer formixing the outputs of the third and fourth multipliers to create thesecond mixed signal.
 8. The apparatus of claim 7, wherein the secondscaling factor is related to the first scaling factor by subtracting thefirst scaling factor from a first predetermined value.
 9. The apparatusof claim 7, wherein the fourth scaling factor is related to the thirdscaling factor by subtracting the third scaling factor from a secondpredetermined value.
 10. The apparatus of claim 1, wherein the digitalmixer comprises a computer processor and a memory havingcomputer-executable instructions stored thereon which, when executed onthe computer processor, cause the computer processor to perform thecombining of a portion of a first audio signal with a portion of asecond audio signal to create the first mixed signal and the combiningof a portion of the second audio signal with a portion of the firstaudio signal to create the second mixed signal.
 11. The apparatus ofclaim 10, wherein: (a) combining a portion of a first audio signal witha portion of a second audio signal to create the first mixed signalcomprises: (i) scaling the first audio signal by a first scaling factor;(ii) scaling the second audio signal by a second scaling factor relatedto the first scaling factor; and (iii) mixing the scaled audio signalsto create the first mixed signal; (b) combining a portion of the secondaudio signal with a portion of the first audio signal to create thesecond mixed signal comprises: (i) scaling the second audio signal by athird scaling factor; (ii) scaling the first audio signal by a fourthscaling factor related to the third scaling factor; and (iii) mixing thescaled audio signals to create the second mixed signal; and (c) themagnitudes of the first and third scaling factors are controlled by thelistener controller in accordance with a listener input.
 12. Theapparatus of claim 11, wherein the second scaling factor is related tothe first scaling factor by subtracting the first scaling factor from afirst predetermined value.
 13. The apparatus of claim 11, wherein thefourth scaling factor is related to the third scaling factor bysubtracting the third scaling factor from a second predetermined value.14. A method for controlling, during playback, the composition of a pairof pre-recorded stereo separated audio signals in a stereo soundentertainment system in accordance with listener input, comprising: (a)receiving two pre-recorded stereo separated digital audio signals; and(b) in response to listener input, (i) combining a portion of a firstdigital audio signal of the two pre-recorded stereo separated digitalaudio signals (“first audio signal”) with a portion of a second digitalaudio signal of the two pre-recorded stereo separated digital audiosignals (“second audio signal”) to create a first mixed signal; and (ii)combining a portion of the second digital audio signal with a portion ofthe first digital audio signal to create a second mixed signal.
 15. Themethod of claim 14, wherein: (a) combining a portion of a first digitalaudio signal with a portion of a second digital audio signal to create afirst mixed signal comprises: (i) scaling the first digital audio signalby a first scaling factor; (ii) scaling the second digital audio signalby a second scaling factor related to the first scaling factor; and(iii) mixing the scaled digital audio signals to create the first mixedsignal; and (b) combining a portion of the second digital audio signalwith a portion of the first digital audio signal to create a secondmixed signal comprises: (i) scaling the second digital audio signal by athird scaling factor; (ii) scaling the first digital audio signal by afourth scaling factor related to the third scaling factor; and (iii)mixing the scaled digital audio signals to create the second mixedsignal.
 16. The method of claim 15, wherein the first scaling factor andthe third scaling factors are controlled by the listener input.
 17. Themethod of claim 15, wherein the second scaling factor is related to thefirst scaling factor by subtracting the first scaling factor from afirst predetermined value.
 18. The method of claim 15, wherein thefourth scaling factor is related to the third scaling factor bysubtracting the third scaling factor from a second predetermined value.19. The method of claim 15, wherein mixing the scaled digital audiosignals to create the first mixed signal comprises adding the firstscaled digital audio signal to the second scaled digital audio signal.20. The method of claim 15, wherein mixing the scaled digital audiosignals to create the second mixed signal comprises adding the thirdscaled digital audio signal to the fourth scaled digital audio signal.21. The method of claim 15, wherein mixing the scaled digital audiosignals to create the first mixed signal comprises subtracting thesecond scaled digital audio signal from the first scaled digital audiosignal.
 22. The method of claim 15, wherein mixing the scaled digitalaudio signals to create the second mixed signal comprises subtractingthe fourth scaled digital audio signal from the third scaled digitalaudio signal.
 23. The method of claim 15, wherein the first and thirdscaling factors are the same.
 24. The method of claim 15, wherein thefirst and second scaling factors are different.
 25. The method of claim15, wherein the second and fourth scaling factors are created bysubtracting, respectively, the first scaling factor from a firstpredetermined value and the third scaling factor from a secondpredetermined value.
 26. The method of claim 25, wherein the first andsecond predetermined values are the same.
 27. The method of claim 25,wherein the first and second predetermined values are one (“1”).